1. Field of the Invention
The present invention relates to a storage protecting apparatus suited advantageously for protecting a main storage of a computer system.
2. Description of the Prior Art
As to the use of a virtual storage in a conventional computer, two methods in general have been known. They are a demand paging method and a segmentation method. For particulars of these methods, detailed discussion will be found in a publication titled "Operating System" by Stuart E. Madnik and John J. Donovan published by McGraw-Hill Company in 1974.
According to the demand paging method, the virtual storage can be effectively allocated to a plurality of data groups or sets without waste. However, with the demand paging method, it is impossible to check in a facilitated manner whether a virtual address of an operand designated by an instruction commanding access to a given data set goes beyond the area allocated to that given data set (this function is referred to as the address boundary check). Thus, the demand paging method suffers a shortcoming in that protection of other data against destruction due to program error can not be readily ensured. On the other hand, the segmentation method with which the address boundary check can be easily effectuated is attended with a problem that a certain part of the virtual storage can not be made use of, thus degrading utilization efficiency of the virtual memory.